Serotonin (5-HT) is a neuromodulator widely expressed in the telencephalon and involved in modulating a number of complex behaviors including learning, emotional state, aggression, and sleep. Improper functioning of the serotonergic system is thought to underlie a number of mental illnesses;understanding the myriad ways in which serotonin affects brain function is thus a crucial goal from both healthcare and basic neuroscience viewpoints. I have chosen to study serotonergic modulation in a premotor song nucleus in the telencephalon of the zebra finch, a model organism which is well-suited for dissecting neuromodulatory actions at multiple levels of organization. Preliminary data reveal a direct action of serotonin on the song system- a strong excitatory effect of 5-HT on the projection neurons of the premotor song nucleus RA, mediated via 5-HT2A (HTR2A) receptors. SSRIs show that endogenous serotonin is sufficient to mediate this affect in vivo, and focal administration of 5-HT during stimulation reveals that these receptors modulate the propagation of electrical signals out of the telencephalon. I propose experiments to verify and extend these findings in terms of both the production and the learning of song. Because of the basic conservation of brain architecture across vertebrates, including the organization of the serotonergic system, what I learn in songbirds will inform our understanding of how serotonin works in other systems, including humans. PUBLIC HEALTH RELEVANCE: Serotonin is a chemical produced in the brain that affects how animals behave- for instance, serotonin has been shown to be associated with depression and also with aggression. I study how serotonin changes the way that birds sing, and also how it changes their ability to learn to sing. What I learn about how serotonin affects birdsong will eventually teach us about how serotonin affects humans.